Oxygen is contained in air in a concentration of approximately 21%, and men breathe and treat fire in such atmosphere.
When the oxygen content in the air rises due to leak of oxygen from an oxygen source into a narrow space or for other reasons, there arise many abnormalities in such atmosphere, such as increased combustibility of clothes and lowered ignition point of the combustibles, resulting in the increased danger of a fire. On the other hand, in the atmosphere with too low oxygen content, man is asphyxiated.
Such excess or scarcity of oxygen can neither be scented nor visible, so that there are many occasions where man enters such locations unaware and gets burnt or suffocates to death.
So, when a person has need of entering a location where such danger is supposed to exist, he must analyze the oxygen content in the air to make sure of safety.
There are available various devices for such use, for example oxygen analyzer utilizing magnetism or oxygen cells or an indicator tube type oxygen sensor. However, these known devices are not necessarily satisfactory in respect of convenience in use at the site and other requirements such as long-time durability, simplicity and quick and safe analyzability.